2 00 7 New Results on Ξ 0 Hyperon Decays

Hyperons have been among the first non-stable elementary particles discovered and are known for more than 50 years now. In spite of this, the physics interest in hyperons is by far not yet exhausted, several important aspects are still under intense investigation. At first, as hyperons only differ by one or two strange-quarks from proton and neutron, they are ideal playgrounds to study SU(3)f symmetry breaking. Secondly, studying their decays offers unique opportunities to understand baryon structure and decay mechanisms. Last, but not least, by comparing hyperon decays to neutron decay, it is possible to measure the CKM matrix parameter |Vus| in a complementary way with respect to kaon decays. However, despite of their interest, most previous measurements date back to the 1960’s and 70’s, in particular those on the neutral Ξ hyperon. Only recently new measurements have been performed, with much increased statistics and leading to a series of new results. The two main new experiments on Ξ decays are the KTeV experiment at Fermilab and NA48/1 at the CERN SPS. Both experiments were designed to measure neutral kaon decays and profit from the fact, that the Ξ lifetime and decay length are of the same order as those of the K S meson. Most of the new results are coming from NA48/1. This experiment was performed in the year 2002 to measure specifically rare K S and neutral hyperon decays. During the data taking period, in total more than 2 billion of Ξ decays took place in the fiducial detector volume, providing enough statistics to precisely measure also very rare Ξ decays.